BEIRUT – Syrian rebels blew up a tunnel packed with explosives in the northern city of Aleppo on Friday, killing at least 20 pro-government fighters, activists and rebels said.

The Britain-based Syrian Observatory for Human Rights activist group said the blast took place near the Zahrawi market not far from the citadel in Old Aleppo. It said clashes followed the explosion.

A powerful rebel alliance called the Islamic Front claimed responsibility for the blast. It said in a tweet that it killed at least 40 government gunmen.

READ MORE: Bombs dropped on Syrian city killed nearly 2,000 this year

The Islamic Front also tweeted a video of the explosion. It shows a massive blast erupting from a skyline of rooftops and satellite dishes, throwing chunks of brick and a huge cloud of dust into the air.

The video appeared genuine and corresponded to other Associated Press reporting.

In early May, rebels also used bomb-packed tunnels to level a historic hotel in the Old City of Aleppo that was being used as an army base.

Such explosions have provided a reminder that the rebels, despite setbacks in other parts of the country, remain a potent force.

Now in its fourth year, Syria’s conflict has killed more than 160,000 people and caused a humanitarian crisis. The United Nations has said that some 9.3 million people – more than 6.5 million displaced by the fighting – are in need of urgent humanitarian assistance inside Syria.

Late Friday, a U.N. spokeswoman said that a 15-truck convoy delivered food aid to Syrian Arab Red Crescent warehouses for 30,000 people in rebel-held areas in western parts of the Aleppo governorate. Stephane Dujarric said the convoy also carried in medicine for 15,000 people, and household items for another 10,000.

“This aid is part of the plan approved last week by the governor of Aleppo to help some half a million people both in opposition and government-held areas,” Dujarric said.

Humanitarian aid has not been able to reach many areas where people are in need, despite a U.N. Security Council resolution in February demanding unfettered access.

Now, Australia, Luxembourg, and Jordan are planning to circulate a new U.N. Security Council resolution that diplomats say would authorize the delivery of humanitarian aid into Syria through four border crossings without approval from President Bashar Assad’s government.

Currently, all U.N. aid must go through Damascus – a practice which U.N. humanitarian chief Valerie Amos has repeatedly criticized.